Sung W. Lee

Cure Simulation of Thick Composite Structures Using the Finite Element Method

A finite element code is developed for the two-dimensional cure simulation of thick composite structures. In contrast to one or two-dimensional codes based on the finite difference method, the present finite element method can easily model composite structures with arbitrary shapes including the mandrel used for the cure set-up. For the two-dimensional cure simulation, a degeneration method to build the thermal conductivity matrix is proposed. The present finite element method is verified by two numerical examples. The numerical results show good agreement with the measured data appeared in the literatures. However, a full three-dimensional cure simulation is needed for more accurate cure simulation.

Time-to-failure of compressively loaded composite structures exposed to fire

A combined thermal-structural analysis methodology based on finite element modeling is developed for the analysis of composite structures exposed to high temperatures due to fire. A simplified heat transfer model proposed by Lattimer is adopted for the thermal response model to determine temperature distributions in composite structures with and without insulation while an assumed strain solid shell finite element formulation is adopted for the structural response model with a material model that takes into account the effects of temperature on modulus and charring. Numerical analyses are carried out to determine the time-to-failure via the global buckling of simply supported and clamped wide columns of an E-glass/ vinylester woven composite material with and without insulation subjected to heat flux and compression. Large deflection non-linear analyses are also conducted to determine the displacement of the columns as the temperature changes. The beneficial effect of insulation on time-to-failure is demonstrated via numerical calculations. Sensitivity analyses are carried out to examine the effect of variation in the glass transition temperature on the time-to-failure of the composite columns.

A Postprocessing Approach to Determine Transverse Stresses in Geometrically Nonlinear Composite and Sandwich Structures

A postprocessing scheme is developed to accurately determine transverse stresses in composite and sandwich panels undergoing geometrically nonlinear deformations. Transverse stresses are recovered at a point of interest via a one-dimensional, equilibrium-based least-square finite element method that utilizes the in-plane stresses and shear forces obtained by a finite element analysis. Numerical results demonstrate that, with minimal addition of computational efforts, the present postprocessing approach can be used to enhance the accuracy of transverse stresses.

Dynamic Analysis of Nonlinear Composite Structures under Pressure Wave Loading

The effect of progressive damages in composite materials is incorporated into a geometrically nonlinear solid shell element formulation to investigate the dynamic response of composite structures under the blast wave pressure loading. The effect of material damages such as fiber failures, matrix cracking, and fiber-matrix shearing failures on the behavior of composite structures is addressed via reducing their stiffness corresponding to detected failures. Two separate failure criteria are used for unidirectional and woven composites. In addition, the strain-rate effect on the material strength is taken into account. For the dynamic formulation, the trapezoidal rule is chosen for the time integration. Various numerical analyses of composite panels are conducted under the static and dynamic loading conditions to validate the effectiveness of the present formulation. The analysis results clearly exhibit the significant effect of material damages on the structural behavior of the composite panels. Also, the analysis results are in good agreement with the available experimental data.

Thermo-viscoplastic modeling of composites exposed to fire or high temperature

This article presents a study on composites exposed to elevated temperatures associated with fire. A thermo-viscoplastic model is proposed for description of the rate-dependent and temperature-dependent behavior of composites at elevated temperatures. AS4/PEEK and E-glass/vinylester composites are chosen to examine the proposed model. Thermo-viscoplastic parameters of the proposed model were determined for each material using experimental data. Off-axis angle tests were performed with various strain rates at various temperatures for E-glass/vinylester woven composite, while experimental data available from literatures were used for AS4/PEEK unidirectional composite. Comparison of the results from the proposed model with experimental data shows good agreement. Also, a few interesting points observed during this study, such as the material behavior near the glass transition temperature, the failure mechanism, and the effect of charring are discussed.

Micromechanical Modeling of Stretch Broken Carbon Fiber Materials

A micromechanical model is developed to characterize the constitutive behavior of stretch broken carbon fiber (SBCF) materials. The model takes into account the characteristics of the viscous resin materials and the effect of randomly distributed discontinuous fibers. The predicted force-to-stretch (FTS) values for uniaxial stretching of SBCF prepreg tapes favorably compare with experimental data.

Analysis of Gossamer Structures Using Assumed-Strain Solid-Shell Finite Elements

The objective of this study is to investigate the applicability of assumed-strain solid-shell finite elements for analysis of gossamer space and near-space structures such as solar sails and scientific balloons. The solid-shell element formulation that alleviates the element locking via the assumed-strain approach has been successfully used for analysis of extremely thin structures. In this study, quadruple precision is explored to further extend the capability of the nine-node assumed-strain solid-shell elements to analyses of extremely thin structures. As examples, geometrically nonlinear static analyses of solar sail ribbons and a square membrane are carried out, without using any stabilizing scheme. Subsequently, the assumed-strain solid-shell element formulation with quadruple precision are successfully used to determine the deformed shape and the stress distribution of a scientific balloon at various altitudes.

Development of Dynamic Force Measurement Capabilities at AEDC Tunnel 9

This work focuses on the development of a hybrid force balance technology that uses piezoelectric sensors in series with conventional strain gauge balance technology to enable broad frequency range force measurements in hypersonic wind tunnel test facilities. Here, sensitivity analyses are carried out to examine the effects of design parameters, including the thickness of mounting plates and location of calibration loading points on the reliability of force and moment measurements.

Mechanics of Curved Pin-Reinforced Composite Sandwich Structures

Pin-reinforced sandwich composites have recently attracted the attention in lightweight structural applications where it substantially improves out-of-plane and shear properties for sandwich composites. However, there is not a great deal of understanding in regards to how shaping these composites affects their mechanical performance when the orientation of the pin-reinforcement may change due to the shaping process. In this investigation, singly curved pin-reinforced sandwich composites using K-Cor have been fabricated using a bend fixture to curve the specimen and heat treatment to soften the core during before bonding with the composite face sheets in order to prevent any damage to the core during the shaping process. Experiments were then performed on the curved sandwich specimens using different boundary conditions on the edges. The boundary conditions were found to result in increased load bearing capacity when supported at the edge compared to support at the bottom due to increased lateral constraint that delayed the onset of bending shear failure. Digital Image Correlation (DIC) was also used to determine the deformation fields from the images captured during deformation to quantify the effects of boundary conditions on the onset of failure initiation, and the results were used to develop a new Finite Element Analysis (FEA) model that is capable predict the mechanical behavior of the curved K-Cor sandwich composites.

Mechanical Characterization and Modeling of X- and K-cor Composites

This research is focused on developing appropriate macro-mechanical models that account for the microstructural details unique to X- and K-cor composite sandwich panels. Digital Image Correlation (DIC) is used to elucidate on the details of the deformation fields within the sandwich structure. These details are used to enhance the models by providing critical details on the failure initiation mechanisms and the subsequent load redistribution that occurs in these structures. The effects of environmental conditions on the mechanical behavior of these composite materials are also being investigated.